reflink: fix quota tests to work properly

reflink: fix quota tests to work properly

Fix the reflink quota tests to su to the fsgqa user so that we actually
test enforcement of quotas.  Seems that XFS enforces user quotas even
if root is writing to a user file, whereas everything else lets root
writes through.  Also clean up some of the variable usage and
_require_user.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>